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The world's first full face transplant, which was authorised by a UK medical watchdog this week, is hedged with technical, psychological and ethical challenges, experts say.

The green light given to surgeon Mr Peter Butler by the ethics committee at London's Royal Free Hospital opens the way to an operation that could take place within months.

But the decision was reached cautiously, reflecting an agonising debate about safety.

These fears were only eased after French doctors last November carried out the first partial face transplant and in July pronounced it a success.

These are the main challenges from a full facial graft, say experts:

Surgery

Nerves and the main vessels that carry blood to the face are connected by surgery under a microscope.

This task is common in reconstructive surgery and has a high success rate. Of the 24 hand transplants that have been carried out on 18 people over the past eight years, only two have failed.

But microsurgery also carries a 5-10% risk of transplant failure from clots that may form within the connected blood vessels in the first few days after surgery.

Immunosupressors

These powerful drugs inhibit the immune system so that the transplanted tissue is not attacked by the body's defences. But they also make the body more vulnerable to infection and disease.

In addition, heavy use of immunosuppressors boosts the risk of cancer. The danger of life-threatening diseases poses an ethical problem because before the transplant, the recipient was generally healthy.

To counter the immunosuppressor problem, doctors may give the graft recipient tiny doses of material from the donor so that his or her immune system becomes accustomed to the tissue and therefore stops attacking it.

The technique is still experimental but has had promising results with kidney transplant patients.

Doctors who performed the partial face transplant on Isabelle Dinoire, 38, last November also gave her a bone marrow graft from the donor.

Psychological

Selecting the patient with the right profile is one of the hardest tasks of all.

The face is the mark of one's identity, so wearing someone else's face, or seeing someone else with the face of a deceased relative, for instance, may be deeply disturbing for some.

But the operation is new, which means no-one really knows for sure how it will work.

On the positive side, computer modelling suggests facial tissue looks quite different and may be even unidentifiable when attached to someone else because the underlying bones and muscles fill it out differently.

"Every graft of a visible organ leads to an identity split, the consequences of which can be very serious if the recipient does not succeed in psychologically accepting the organ and in rebuilding its social expression in everyday life," French experts wrote in The Lancet in July.

The right candidate is therefore someone who is robust and does not entertain the belief that the operation will miraculously restore their former lives, a hope that fades to bitterness and alienation when complications inevitably occur.

Seen as 'alien'

In the case of the world's first hand transplant, recipient Clint Hallam begged to have the new limb cut off because he viewed it as alien.

"Thorough psychological preparation is vital to the successful selection of patients," says The Face Trust, a charity set up to fund research for surgical reconstruction and face transplants.

"The challenge we face is that the people who cope least well with facial injury may also be the [people who] cope least well with facial transplantation and its long-term impact on lifestyle."